My branche is called 1.0.0-dev and was therefore excluded. Added that, and now it works. Build is failing with hundreds of code-formatting issues, but those are easily sorted. After that, it fails at the EPV. Some warnings and this error:

Error: Packaging structure doesn't meet the extension DB policies.

Not sure what's wrong with it though, since the structure comes from the skeleton-tool...

FILE: ...ome/travis/build/phpBB3/phpBB/ext/ger/cmbb/cmbb/presentation.php
----------------------------------------------------------------------
FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
----------------------------------------------------------------------
1 | WARNING | A file should declare new symbols (classes, functions,
| | constants, etc.) and cause no other side effects, or it should
| | execute logic with side effects, but should not do both. The
| | first symbol is defined on line 24 and the first side effect
| | is on line 16. (PSR1.Files.SideEffects.FoundWithSymbols)
----------------------------------------------------------------------

But that's because phpBB requires the IN_PHPBB check in that file since it's only a collection of functions.

Is this even the reason for the build failure or has it nothing to do with it? Besides this one, all other jobs pass so I'd like to have this one passing as well.

I understand and quite honestly it wouldn't be that difficult (just a bit of needless overhead IMO). But if that's the case, phpBB would have to rebuild every file that's directly in the ./includes folder...
Also the documentation should mention that instead of advising it the way I have done it currently.

But hey, if this is needed to get my green badge from Travis I'll wrap it in a class.

Well, rewriting includes is in progress, but that is not something done within a year . The goal is to move away completely from the includes folder, and a bunch of those methods in there is already deprecated.

The overhead of using a helper class is not that much, and the code itself will be a lot cleaner.

I know, but this is just for starters. The extension is in such an early dev state now it's way to soon to start writing tests.
Just want to make sure that when I do start writing tests, results aren't clouded with basic stuff. It also helps writing clean code.

If you're following the guide and included all of the PHP versions listed there then Travis will run your tests on those different PHP versions. If you don't want tests run using certain PHP versions and/or some DBMS types then just remove them from your .travis.yml file.